The present invention relates to a photovoltaically self-charging storage system having a layer structure in which a first material, which forms a first redox system, a second material, which forms a second redox system, and a photosensitive material are arranged in or between a first and a second electrically conductive layer. This system is a multi-layer system which stores electric energy electrochemically when illumuniated and can release electric energy at a later time to operate an electrical consumer.
For the mentioned application of operating an electrical consumer, the state of the art presently provides photovoltaic systems which release the required energy directly to the consumer. If the energy is not needed until later, these systems require an additional battery for storage.
Furthermore, DE 29 24 079 discloses an arrangement in which a p-n junction is employed in conjunction with a solid electrolyte to generate a charge and to store it. Such type systems, however, have the disadvantage that they are relatively complicated in structure and the selection of material is greatly limited.
The object of the present invention is to provide a photovoltaic self-charging storage system which is of simple construction and permits storing the energy provided for the operation of an electrical consumer photovoltaically.
This object is solved with the photovoltaic self-charging storage system of the valid claim 1. Advantageous embodiments and further developments of this system are the subject matter of the subclaims.
A key element of the present invention is that the photovoltaically self-charging storage system has a layer structure, in which a first material, which forms a first redox system Aox/Ared, a second material, which forms a second redox system Box/Bred, and a photosensitive material are arranged in or between a first and a second electrically conductive layer, with at least one of the electrically conductive layers being transparent for visible light. The first and the second material are each in electrical contact with one of the electrically conductive layers. Furthermore, the first material, the second material and the photosensitive material are selected and arranged in the layer structure in such a manner that, due to the effect of light, the photosensitive material releases electrons to the first redox system and picks them up again from the second redox system, thereby inducing a redox reaction Aox+Bredxe2x86x92Ared+Box (direct reaction), which can be reversed (back reaction) if light ceases, and with the back reaction being substantially slower than the direct reaction. Someone skilled in the art is familiar with combinations of materials suited for this purpose. The thickness of the layer over which the first and/or second material extends is configured in such a manner that a charge storage in the layer structure yielded by the direct reaction and in its capacity sufficient for operating an electrical consumer is made possible.
The invented storage system combines, in an advantageous manner, in a layer structure, a photovoltaic element with an electrochemical storage. A photosensitive material, preferably a coloring matter and two redox pairs Aox, Ared and Box, Bred are combined in a suited manner and arranged between two electrically conductive layers. One of the materials can, of course, also itself form the electrically conductive layer respectively be embedded in the electrically conductive layer. Charging the storage system occurs by means of the reaction Aox+Bredxe2x86x92Ared+Box initiated by the photosensitive material (for example: coloring matter) under light. The coloring matter (F) is excited by the light and can release an electron exe2x88x92 directly or via a semiconductor to Aox, leading to a reduction of Aox. For charge compensation of the coloring matter, the latter picks up an electron from Bred (directly or indirectly) and thereby oxidizes Bred. These processes can be expressed by the following formulae:
F+photonxe2x86x92F*(excitation coloring matter)xe2x80x83xe2x80x83(1)
F*+Axe2x86x92F*(rapid injection of exe2x88x92 from coloring matter to A; oxidation of A)xe2x80x83xe2x80x83(2)
F++Bxe2x86x92F+B+(injection of e31  from B to the coloring matter; reduction of B)xe2x80x83xe2x80x83(3)
The function principle of the storage system is that the photosensitive material injects under light energetically excited charge carriers directly or indirectly via another material (see, e.g. claim 4) into the redox pair A and leads to a redox reaction there. In the case of the electron injection, A is reduced. From redox pair B, the photosensitive material receives a charge carrier again. In the case of electron injection, B is oxidized.
Depending on the layer structure respectively the layer sequence (see, e.g. claims 6, 9 and 13), Ared and Box can, for examplexe2x80x94in the case of electron injection by means of the photosensitive materialxe2x80x94lie directly adjacent and form an electrochemical double layer or a counter ion to one of the components of a redox pair passes over to the other and in this way compensates the charge transfer by means of the photosensitive material. In the latter case, intermediate layers possibly present between the redox pairs have to be permeable for the counter ion.
Besides this direct reaction, in redox reactions, the back reaction constantly occurs simultaneously. However, it should be greatly checked by the selection of materials. For functioning of the invented system as a storage system, a slow back reaction rate is required in the region in which the direct reaction occurs. Someone skilled in the art knows numerous materials which possess these properties. For example, a combination of WO3 or TiO2 as the first redox system combined with inorganic ruthenium compounds as photosensitive material and with LiI in propylene carbonate as the second redox system demonstrates a very slow back reaction rate.
The back reaction is a prerequisite for the discharge of the system. A slow back reaction permits operation of only a consumer with very low current requirements. Suited external circuitry of the system combined with a suited layer structure, as will be described in more detail in the following, can result in spatial separation between the direct reaction and the back reaction. If a catalyst material is provided at the site of the back reaction for the back reaction in the layer structure, the back reaction can be substantially accelerated depending on the external circuitry and thus supply greater current strengths. Even tiny additives of platinum act as a catalyst on the back reaction of the just described redox systems.
Functioning of the invented storage system requires that the layer thickness, over which the first or the second redox system extends, is great enough to be able to store sufficient amount of charge for the operation of an electrical consumer. Preferably layer thicknesses of 0.01 to 1 mm or more are employed.
Thus, with the photovoltaic storage system according to claim 1, the light energy is photovoltaically converted and stored in a simple layer structure to operate an electrical consumer immediately with the energy or at some later time. Besides the simple structure, this system has the further great advantage over the state-of-the art systems that, an additional battery is no longer needed for charge storage.
Materials like those used for coloring-matter-sensitized solar cells , electrochromic systems and electrochromic coloring-matter-sensitized solar cells can be employed for the layer structure. These systems are optical components which reversibly alter their optical or electrical properties by means of illumination or external circuitry. Such an optical component is, for example, described in xe2x80x9cPhotoelectrochromic Window and Displaysxe2x80x9d, C. Bechinger et.al., Nature, Vol. 383, 17 October 1996, pp. 608 to 610. In this system, the layer structure is realized by two redox systems and a coloring matter, which is similar to the present invention. Opening or closing one of the external circuits permits retaining or reversing the coloration of an electrochromic WO3 layer inside the layer structure.
The thickness of less than 1 xcexcm of the provided layers there, however, makes charge storage for operation of an electrical consumer impossible. Substantially thicker layers would be required.
The materials employed in the present system can be arranged as a mixture or superimposition in the form of different layers in the layer structure. Further more, additional materials can be mixed into these materials or provided as an additional layer in the layer structure. Such types of additional materials, for example for improving the bonding of the photosensitive materials to the first redox system or for increasing the velocity of the back reaction, can enhance the kinetics of the processes and the stability of the system.
TiO2 or WO3 as Aox, and I3xe2x88x92 and Ixe2x88x92 as Box and Bred can be given as examples of materials which form the redox systems. Redox system B can, for example, be present in the form of dissolved LiI and I2  in an acetonitrile layer. The photosensitive materials are, for example coloring matters like those utilized in coloring-matter-sensitized solar cells, e.g. inorganic ruthenium compounds, anthocyanins, chlorophylls or perylene coloring matters.
The single systems can, as previously described, be directly adjacent layers and form an electrochemical double layer.
In another embodiment, it can be provided that in the redox reaction, a counter ion to a component of a redox system can pass from the phase of this redox system to the phase of another redox system and in this manner compensate the charge transfer by the photosensitive material.
An example of this is a nanoporous WO3 layer as the first redox system onto which a monolayer of an inorganic ruthenium compound is applied as the photosensitive material (coloring matter). The pores of the WO3 layer are saturated by an electrolyte containing LiI and LiI3 as salts (second redox system). When exposed to light the coloring matter oxidizes Ixe2x88x92- to I3xe2x88x92, whereas the Li+ ion migrates into the WO3, which contains an additional electron from the coloring matter. The second redox reaction then is W(+6)O3+Li++exe2x88x92xe2x86x92LiW(+5)O3. In order to improve the bonding of the coloring mattering to the first redox system, the coloring matter can be applied onto an additional TiO2 layer disposed on the WO3.
In a preferred embodiment of the present storage system, the first material, the second material and the photosensitive material form different layers which are arranged in a layer sequence of electrically conductive layer, first material, photosensitive material, second material, optionally a catalyst layer, and a second electrically conducting layer. Optionally an intermediate layer can be provided between the first material and the photosensitive material, for example to improve the bonding between the photosensitive material and the first material. One of the materials forming the redox system possesses good conductivity, preferably of greater than 0.01 (xcexa9cm)xe2x88x921; the other poor conductivity of less than 10xe2x88x924 (xcexa9cm)xe2x88x921. This prevents the current from flowing via an externally connected consumer, but rather internally between the two electrically conducting layers.
In a second preferred embodiment, a layer structure is provided in which a layer sequence is realized with an electrically conductive layer, optionally an intermediate layer, the photosensitive material, optionally a catalyst layer, the second material, the first material and another electrically conducting layer. In this case, too, one of the materials forming the two redox materials is electrically good conducting and the other electrically poor conducting. Preferable conductivity values have been given in connection with the preceding embodiment. Therefore in both embodiments, a spatial separation between the direct reaction and the back reaction, which permits conducting the current via an external consumer, is achieved by means of a suited layer structure.
Furthermore, materials that alter their optical or electrical properties with the charge state can be utilized as materials. These are materials like those, for example, used for coloring-matter-sensitized solar cells. In this way, the additional effects of theses known optical systems can also be utilized in the present system.